Over the past few decades, block copolymers have emerged as a class of soft materials with a wide range of technological applications. Due to the high tunability of their chemical structure (i.e., morphology, architecture and domain size), block copolymers have been utilized as surfactants, thermoplastic elastomers, nano-templates, membranes, etc. Controlled polymerization, such as ionic, controlled radical (i.e., ATRP, NMP, RAFT), and ring opening polymerization have been the standard means utilized for producing block copolymers. While these methods have proven useful, they lack the ability to homopolymerize the world's two most produced and inexpensive monomers: ethylene and propylene.
Polyolefins are generally produced industrially via catalytic insertion (co)polymerization of ethylene, propylene, and linear α-olefins on the scale of 70×106 metric tons per year. The tunability of the polymer's crystallinity offers a mixture of properties, such as, toughness, elasticity, solvent resistance, etc., which are difficult to reproduce economically by other monomers. Therefore, the incorporation of polyolefins into block copolymers would be of significant value, as it would further expand the usefulness of the largest family of polymers in our society. However, due to the high oxophilicity of the insertion metal catalysts used in industry, commercial polyolefin block copolymers have been limited to apolar monomers, which also limits applications. Hence, the synthesis of functionalized block copolymers containing polyolefins remains a modern challenge for synthetic chemists.
Over the years, alternative approaches have been developed to synthesize functionalized polyolefin block copolymers. Most approaches rely on the formation of (semi-) telechelic polymers, which can be used to introduce a polar functionality. One of the most popular of these approaches employs the anionic polymerization of butadiene quenched by an epoxide, followed by the hydrogenation of the polymer to yield a hydroxyl terminated linear low density polyethylene (LLDPE) block. This approach may be efficient, but it is not compatible with the synthesis of polypropylene blocks and uses stoichiometric amounts of a pyrophoric initiator. Ring opening metathesis (ROMP) of cyclic alkenes has also been implemented to yield telechelic high density polyethylene (HDPE). Despite the success of these approaches, employing monomers such as butadiene and cyclic alkenes remains less favorable than the direct polymerization of ethylene. Furthermore, the inability of these approaches to yield block copolymers containing stereoregular polypropylene (PP) is limiting. Living insertion polymerizations have also been reported to produce telechelic polyolefins from ethylene and propylene. However, these systems only produce one polymer chain per metal, which drastically limits their commercial potential. Chain transfer insertion polymerizations have also been reported to yield multiple chains of polymer per catalytic site. However, this approach still requires high loadings of metal(loid) chain transfer agents and also provides low stereocontrol.
Some groups have employed postpolymerization modification techniques to convert vinylic terminated polyolefins into macroinitiators. Hydrosilation, thiol-ene, hydroalumination and esterification reactions have been applied with moderate to high conversions. It is worth noting that these reactions were only performed on low molecular weight vinylic terminated polyolefins (Mn<5 kg/mol). The non-quantitative conversion of most of these reactions and the exclusive reactivity toward the vinyl terminated polymers (which is catalyst dependent and often not the most common end-group) drastically restricts the impact of these previous methods. Additionally, in-situ cross coupling compatiblization has been recently reported resulting in a process with less than 50% efficiency for producing the desired block copolymer.
The approach described in this patent aims to address these limitations by offering a more universal and versatile platform that quantitatively converts mono and substituted (e.g., disubstituted) alkene terminated polyolefins of essentially any molecular weight into block copolymers.